If you look in the centre of V3 and V5 on the pic I posted, you will see tallish black things, with twisted yellow wires going from the light bulb, to the splicer on V3, then the splicers on V5.

The splicer itself has a small blade that cuts the wire open when you insert it in and push it on the splicers recipient 3 shafts (those shafts/forks/male parts whatever you call it are made of of metal and make the connection with the rest of the PCB circuit). Essentially, they mean cheaper assembly costs, but at the expense of reliability in my experience.

I believe Orange has been moving away for these splicers in more recent products. On the plus side, they're a reasonably easy fix - Hardwire them and you'll never have that issue again.

Here I think the heater splicers on V5 is connected to the transformer supplies on TX4 and TX6, passes on the valve heater power supply to V4 via PCB traces, and to V3 and the bulb via the splicers.

My guess is that the connection fails on the splicer on V5 - otherwise both EL34 would warm up.

Or the other thing to do is to measure the AC voltage at the spade connectors where the thick yellow wires attach - TX4 & TX6.
If there is no voltage here with the amp switched on then the 6.3V tap is toast and it is new power transformer time.